Advanced microelectronic heat dissipation package and method for its manufacture

ABSTRACT

Heat dissipation during the operation of integrated circuit chips is an old problem that continues to get worse. The present invention significantly ameliorates this by placing an embedded heat pipe directly beneath the chip. Using powder injection molding, the lower portion of the package is formed first as an initial green part which includes one or more cavities. The latter are then lined with a feedstock that is designed to produce a porous material after sintering, at which time a working fluid is introduced into the porous cavities and sealed, thereby forming one or more heat pipes located directly below the chip. The latter is then sealed inside an enclosure. During operation, heat generated by the chip is efficiently transferred to points outside the enclosure. A process for manufacturing the structure is also described.

FIELD OF THE INVENTION

The invention relates to the general field of microelectronics withparticular reference to packaging and heat dissipation.

BACKGROUND OF THE INVENTION

As the world moves towards higher performance communication and computersystems, microelectronic devices are running into basic barriers relatedto heat dissipation. Options under exploration include active andpassive packaging designs. The active designs require fans or pumps tocirculate fluids for heat extraction, often leading to heat generation,power drain, and new failure modes. The alternative is to move intopassive designs, such as fins to radiate heat into the enclosure.

High thermal conductivity materials are desirable for heat dissipation,and current favorites include tungsten-copper, molybdenum-copper, andaluminum or copper. The latter choices suffer from high thermalexpansion coefficients which induce a new failure mechanism throughthermal fatigue, associated with turning on (heating up) and turning off(cooling down) an electronic device. In order to sustain the desiredthermal expansion match with silicon while maximizing thermalconductivity, the top materials then tend to be heavy, expensive, andmodest in thermal conductivity. Only diamond provides a high thermalconductivity with low thermal expansion, but its cost is prohibitive.

In recent years, we have made progress in designing improvedfunctionality into a structure through the combination of two differentmaterials using a process termed two-material powder injection molding.This step toward functionality directly built into a device haspotential benefits in microelectronic packaging. The walls might befabricated from a good glass-sealing alloy, such as kovar, while thebase would be fabricated from a low thermal expansion material, such astungsten-copper. However, even these two-material combinations arelimited by the thermal conductivity of the base. Currently,tungsten-copper is capable of thermal conductivities in the 200 W/m/Krange. This is still half that possible with pure copper, but, again,still fails to satisfy the thermal expansion requirement. We note thatdiamond can achieve 2000 W/m/K.

As will become clear later, the present invention makes use oftwo-material powder injection molding to implement a different approachto this problem. This process has been described in application Ser. No.09/733,527 Dec. 11, 2000 “Method to form multi-material components”.Briefly, this process shows how powder injection molding may be used toform a continuous body having multiple parts, each of which hasdifferent physical properties such as magnetic characteristics orhardness. This is accomplished through careful control of the relativeshrinkage rates of these various parts. Additionally, care is taken toensure that only certain selected physical properties are allowed todiffer between the parts while others may be altered through relativelysmall changes in the composition of the feedstocks used.

A routine search of the prior art was performed and the following USpatents were found to be of interest:

U.S. Pat. No. 6,410,982 (Brownell et al.); U.S. Pat. No. 6,321,452(Lin); U.S. Pat. No. 6,385,044 (Colbert at al); U.S. Pat. No. 6,370,749(Tseng et al.); U.S. Pat. No. 6,303,191 (Henne et al.); U.S. Pat. No.6,293,333 (Ponnappan et al.); U.S. Pat. No. 6,230,407 (Akutsu); and U.S.Pat. No. 6,070,654 (Ito).

Additionally, the following publications were discovered during oursearch:

1. B. R. Babin, G. P. Peterson, and D. Wu, “Steady-State Modeling andTesting of a Micro Heat Pipe,” Journal of Heat Transfer, vol. 112,August 1990, pp. 595-601.

2. J. P. Longtin, B. Badran, and F. M. Gerner, “A One-Dimensional Modelof a Micro Heat Pipe During Steady State Operation,” Journal of HeatTransfer, vol. 116, August 1994, pp. 709-715.

3. L. W. Swanson, “Heat Pipes,” The CRC Handbook of Thermal Engineering,F. Kreith (ed.) CRC Press, NY, 2000, pp. 4.419-4.429.

SUMMARY OF THE INVENTION

It has been an object of at least one embodiment of the presentinvention to provide a heat pipe that can be cheaply produced andreadily miniaturized.

Another object of at least one embodiment of the present invention hasbeen that said heat pipe be readily made part of a package suitable forhousing, and rapidly removing, heat generated by a semiconductor chip.

Still another object of at least one embodiment of the present inventionhas been to provide a process for manufacturing said heat pipe.

A further object of at least one embodiment of the present invention hasbeen to provide a process for manufacturing said chip package.

These objects have been achieved by placing an embedded heat pipedirectly beneath the chip. Using powder injection molding, the lowerportion of the package is formed first as an initial green part whichincludes one or more cavities. The latter, if their dimensions exceedabout 1,000 microns, are then filled with a feedstock that is designedto produce a porous material lining after sintering. Cavities withdimensions less than about 1,000 microns may be left unfilled. Aftersintering, a working fluid is introduced into the cavities and sealed,thereby forming one or more heat pipes located directly below the chip.The latter is sealed inside an enclosure. During operation, heatgenerated by the chip is efficiently transferred to points outside theenclosure. A process for manufacturing the structure is described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of the structure of the present invention.

FIGS. 2 and 3 are cross-sections through two different planes of thestructure seen in FIG. 1.

FIG. 4 is an isometric representation of FIGS. 1, 2, and 3.

FIG. 5 is a flow chart summarizing the principal steps of the process ofthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention discloses a novel functional design that ismaterial based on selective porosity embedded at desired locations in amicroelectronic package. These are placed directly in themicroelectronic package under the semiconductor chip to afford thegreatest heat dissipation. The purpose of the selective porosity is toenable wicking of the condensed working fluid, such-as alcohol or water,from a cold region back to a hot region. The liquid phase evaporatesfrom the hot region, thereby consuming the enthalpy (heat content) ofevaporation. The vapor now migrates through a channel to a cold regionwhere it redeposits the enthalpy via condensation. Such behavior is wellknown as heat pipe technology, such heat pipes being used as structuresfor the transport of heat via evaporation and condensation of a workingfluid in a porous pipe or wick. Heat pipes can deliver from 50,000 to200,000 W/m/K in equivalent thermal conductivity.

Structures of the type described above are relatively expensive and notwell suited for incorporation in a microelectronic packaging scheme. Thepresent invention teaches how two-material injection molding technologycan be applied to the fabrication of high heat dissipationmicroelectronic packages. This is achieved by first injection moldingthe housing with cavities located in appropriate places. One means forachieving this goal would be to use a sacrificial material insert in themold, but a preferred means is to use. the two-color metal powderinjection molding technology already known to AMT.

In this latter case, hard tooling inserts form the cooling patternduring the first molding. In the second stage of injection molding, alow packing density powder feedstock is injection molded into thepattern of the heat pipe located in the overall package. Controlledporosity and pore size are possible by selection of the proper metalpowder size and powder to polymer ratio. Additional control can beachieved by inclusion of sacrificial particles in the feedstock. Thelatter route provides precise control over the pore size and porosity,independent of the metal powder size, since the concentration and sizeof the sacrificial particles are independently controlled when batchingthe feedstock.

After the second step in injection molding, the component is subjectedto normal debinding and sintering cycles. After sintering, the secondmaterial regions will be incompletely sintered, leaving behind thedesired porosity and pores for the eventual creation of high thermalconductivity heat pipes. A working fluid is then dosed into the poresand the pore channels sealed by solder, plugging, or other mechanical orstandard means. Since heat pipes dissipate 100 times more heat thandiamond, the resulting microelectronic packages can far exceed what canbe attained with current designs.

The approach taken by the present invention allows the fabrication ofmore complicated patterns, including curved and conformal coolingpassages. Additionally, the present invention allows the fabrication oflightweight devices from aluminum or other low-density materials forportable devices, such as cellular telephones and laptop computers.

We will describe the present invention in terms of a process for itsmanufacture. This description will also further clarify the structure ofthe present invention. Referring now to FIG. 1, shown there is a planview of the structure. Its principal parts are a solid body 11 withinwhich there are cavities such as 12. These cavities have been lined withporous material 15 which is saturated with a working fluid. The unfilledportions of the cavities are open vapor channels.

The manufacturing process starts with the preparation of a feedstockmade up of particles of the material from which solid body 11 is to beconstructed, as well as a suitable binder (and possibly, for somematerials, a fluxing agent). Suitable materials for these particlesinclude, but are not limited to, copper, aluminum, tungsten,tungsten-copper, kovar, stainless steel, or nickel alloys. The particlesare carefully chosen for their ability to sinter density to near 100% ofthe materials theoretical density. Generally, this requires a meanparticle size less than 30 μm, but is material dependent. For example,tungsten-based materials generally require a mean particle size less 5μm. Suitable binder materials include combinations of waxes, organicpolymers, and surface active agents, such as stearic acid. These bindersare melted and the particles are uniformly dispersed within them.Typically, the particles compose 50-65 percent of the total volume offeedstock while the binder composes the rest.

Prior to injection of the feedstock, inserts are added to the mold.These have the shape of the cavities 12 that are to be formed and areplaced in the appropriate locations. At this stage, the cavities willalways have at least one open end although more complicated designs mayrequire two or more open ends per cavity. Typically, a simple straightcavity as exemplified by 12 in the figures would be between about 2 and8 mm wide, 2 and 5 mm deep, and between 12 and 60 mm long. The insertsmay be made of sacrificial materials such as organic polymers, includingpolyethylene, polypropylene, and polystyrene, so that the cavity isformed when they are removed through liquefaction, vaporization, orchemical action, or they may be made out of materials, such as bronze,steel, or alumina, which allows them to be removed intact for laterreuse. The latter type of insert is to be preferred, for economicreasons, unless the shape of the cavity is such that the insert cannotbe removed without changing the cavity's shape. With the feedstockheated and the insert(s) in place, molding under pressure takes place,resulting in the formation of the initial green body.

Cavities with dimensions greater than about 1000 μm in the green bodyare now partially filled with a second feedstock which was previouslyprepared from particles having a mean diameter between about 40 and 200μm, uniformly dispersed within a suitable binder. Suitable materials forthese particles include, but are not limited to copper, aluminum,tungsten, tungsten-copper, kovar, stainless steel, or nickel alloys. Thelarger size of the particles here ensures that the material that will beobtained after sintering will be porous. An additional degree of poresize control may be achieved by adding to the second feedstock particlesof a sacrificial material such as graphite, which evacuates thestructure during sintering, leaving behind voids. Typically, the volumefraction of the non-sacrificial particles in the mix is 30-40 percent ofthe total volume of the feedstock.

Prior to injection of the second feedstock, a second set of inserts areadded to the green body. These have the shape of the vapor channelswithin the cavities 12 that are to be left open. They may be made out ofthe same types of sacrificial or reusable materials as the first set ofinserts. With the second feedstock heated and the initial green bodywith insert(s) in place, a second molding under pressure takes place,resulting in the formation of the final green body. Cavities withdimensions less than about 1000 μm can be left unfilled.

The process now continues with routine removal of all binding materialsfrom the final green body resulting in the formation of a skeletonstructure. The latter is then sintered to form the final body. The stepof sintering consists of heating at a temperature and time and in anatmosphere that depends on the composition of the body. For example, acopper body is sintered at a temperature between about 900 and 1070° C.for between 5 and 120 minutes in an atmosphere of hydrogen. Note that animportant feature of the process (though not part of the presentinvention) is the control of the nature and concentration of the variousbinders used to form the feedstocks so as to ensure that porous anddense portions of the structure shrink by the same amount duringsintering. Details about this aspect of the invention can be found inthe aforementioned application Ser. No. 09/733,527 filed Dec. 11, 2000.

The next step is the introduction of a working fluid into the cavity sothat it may function as a wick. The working fluid must have a triplepoint below its operational temperature and a critical point above itsoperational temperature. Typically, the working fluid will have a triplepoint below 20° C. and a critical point above 100° C. Examples ofliquids suitable for use as a working fluid include, but are not limitedto, water, ammonia, acetone, and alcohol. The working fluid occupiesenough of the cavity volume to saturate the wick. This is accomplishedby first evacuating the cavity under vacuum, charging the cavity withthe fluid, then sealing all open ends with plugs 13, thereby forming theheat pipe. Possible ways to seal the cavities include, but are notlimited to, epoxying, welding, crimping, soldering, and press fitting.

Enclosure 14 is then formed on the upper surface of body 11. This isbest seen in FIG. 2 which is a cross-section taken through 2-2 inFIG. 1. Enclosure 14 is placed directly over one or more of the heatpipes. Cross-section 3-3 can be viewed in FIG. 3 which also shows thedirection of heat flow. Starting at hot spot 31 (generally the undersideof a chip) heat is transferred by conduction through solid body 11 tothe porous wick 32 where it causes the working fluid to evaporate andexpand along the vapor space in directions 33, carrying the heat withit. Once the hot vapor passes outside the sphere of influence of hotspot 31 it condenses back to liquid, as symbolized by arrows 34. As theliquid concentration builds up there, capillary forces draw it backthrough the porous wick 32 toward hot spot 31 where the cycle can beginagain.

The process concludes with the step of mounting the chip on the uppersurface of body 11, using a high thermal conductivity medium, such assilver-filled epoxy, followed by sealing of enclosure 12 (with the chipinside it). Optionally, a high conductivity material such as helium maybe sealed inside 14 together with the chip. FIG. 4 is an isometric viewof the structure showing a chip 41 in place inside enclosure 14. Notshown in FIG. 4 are contact pads, formed inside enclosure 14 to receivechip 41, together with leads, connected to these contact pads thatextend away from the chip to terminate outside the enclosure.

EXAMPLE

A 99.85% pure copper powder with a mean particle size of 15 μm is mixedwith an organic binder composed of paraffin wax, micropulver wax,polyethylene, and stearic acid to form a first feedstock. Said 15 μmcopper powder comprises 50 volume percent of the said first feedstock. A99.88% pure copper powder with a mean particle size of 50 μm is mixedwith the same binder composition to form a second feedstock. Said 50 μmcopper powder comprises 35 vol. % of the said second feedstock. Thefirst feedstock is molded into the shape of a cylindrical housingmeasuring 18 mm long and 12.5 mm in diameter.

A cylindrical steel insert forms a cavity measuring 16 mm long and 5 mmin diameter. A 16 mm long and 3 mm in diameter insert consisting ofpolyethylene and 20% paraffin wax is then added to the cavity of thegreen housing and the second feedstock is molded into the open portionof the cavity. The green body is freed of all organic binder by heatingin a controlled furnace over a period of 25 hours at 600° C. in ahydrogen atmosphere. The debound body is heated to 1050° C. at a rate of350° C./hr in a hydrogen atmosphere. After sintering at 1050° C. for twohours, the furnace is allowed to cool. This results in a housing with adensity of about 8.6 g/cm³, which is close to its theoretical density,and a porous layer within the housing with a density of about 60% oftheoretical.

Good bonding is observed between the porous layer and housing surface.This housing can then be charged with water and sealed for operation asa heat pipe for removing heat from a semiconductor chip, placed in anenclosure on one end of the cylindrical structure. The effectiveness ofthis design for power dissipation was confirmed by calculations thatshow that, at a nominal operating temperature of 40° C., the package iscapable of transporting over 100 W of waste heat away from the chip. Aflow chart for the above process in presented in FIG. 5.

1. A process to manufacture a heat pipe, comprising: providing a first feedstock, having a first volume, that further comprises first particles, having a mean diameter less than about 30 microns, uniformly dispersed within a first binder, said first particles occupying between about 50 and 65% of said first volume; providing a second feedstock, having a second volume, that further comprises second particles, having a mean diameter between about 40 and 200 microns, uniformly dispersed within a second binder, said second particles occupying between about 30 and 40% of said second volume; molding said first feedstock around a first insert to form a green body; then removing said first insert from the green body thereby forming, in the green body, a cavity having one closed end and one open end; adding a second insert to said cavity; filling said cavity with said second feedstock; removing all binding materials from said green body and from said second feedstock inside said cavity, thereby forming a first skeleton within which is a second skeleton that is more porous than said first skeleton; sintering said first and second skeletons, thereby forming a dense body that includes an interior wick; introducing a working fluid into said wick; and then sealing said open end, thereby forming said heat pipe.
 2. The process described in claim 1 wherein said first particles are selected from the group consisting of copper, aluminum, tungsten, tungsten-copper, kovar, stainless steel, and nickel alloys.
 3. The process described in claim 1 wherein said second particles are selected from the group consisting of copper, aluminum, tungsten, tungsten-copper, kovar, stainless steel, and nickel alloys.
 4. The process described in claim 1 wherein said second particles are removed from within said cavity during the step of sintering the skeletons.
 5. The process described in claim 1 wherein the step of removing said first insert from the green body further comprises destruction of said first insert.
 6. The process described in claim 1 wherein said first insert is selected from the group consisting of bronze, steel, and alumina.
 7. The process described in claim 1 wherein said first insert may be reused after the termination of said process.
 8. The process described in claim 1 wherein said working fluid is selected from the group consisting of water, ammonia, acetone, and alcohol.
 9. A process to manufacture a heat pipe, comprising: providing a feedstock, having a volume, that further comprises particles, having a mean diameter less than about 30 microns, uniformly dispersed within a binder, said particles occupying between about 50 and 65% of said volume; molding said feedstock around an insert to form a green body; then removing said insert from the green body thereby forming, in the green body, a cavity having one closed end and one open end and dimensions less than about 1,000 microns; removing all binding materials from said green body, thereby forming a skeleton within which is a micro-channel capable of functioning as a wick; sintering said skeleton thereby forming a dense body that includes said wick; introducing a working fluid into said wick; and then sealing said open end, thereby forming said heat pipe.
 10. A process to manufacture a package containing a heat pipe, comprising: providing a first feedstock, having a first volume, that further comprises first particles, having a mean diameter less than about 30 microns, uniformly dispersed within a first binder, said first particles occupying between about 50 and 65% of said first volume; providing a second feedstock, having a second volume, that further comprises second particles, having a mean diameter between about 40 and 200 microns, uniformly dispersed within a second binder, said second particles occupying between about 30 and 40% of said second volume; molding said first feedstock around a first insert to form a green body; then removing said first insert from the green body thereby forming, in the green body, a cavity having one closed end and one open end; adding a second insert to said cavity; filling said cavity with said second feedstock; removing all binding materials from said green body and from said second feedstock inside said cavity, thereby forming a first skeleton within which is a second skeleton that is more porous than said first skeleton; sintering said first and second skeletons, thereby forming a dense body that includes an interior wick; introducing a working fluid into said wick; then sealing said open end, thereby forming said heat pipe; on a surface of said package, forming an open enclosure that is overlapped by said heat pipe; then mounting said integrated circuit chip inside said enclosure on said surface using a high thermal conductivity medium; and then sealing said enclosure.
 11. The process described in claim 10 wherein said first particles are selected from the group consisting of copper, aluminum, tungsten, tungsten-copper, kovar, stainless steel, and nickel alloys.
 12. The process described in claim 10 wherein said second particles are selected from the group consisting of copper, aluminum, tungsten, tungsten-copper, kovar, stainless steel, and nickel alloys.
 13. The process described in claim 10 wherein said second particles are removed from within said cavity during the step of sintering the skeletons.
 14. The process described in claim 10 wherein the step of removing said first insert from the green body further comprises destruction of said first insert.
 15. The process described in claim 10 wherein said first insert is selected from the group consisting of bronze, steel, and alumina.
 16. The process described in claim 10 wherein said first insert may be reused after the termination of said process.
 17. The process described in claim 10 wherein said working fluid is selected from the group consisting of water, ammonia, acetone, and alcohol.
 18. A process to manufacture a package containing a heat pipe, comprising: providing a feedstock, having a volume, that further comprises particles, having a mean diameter less than about 30 microns, uniformly dispersed within a binder, said particles occupying between about 50 and 65% of said volume; molding said feedstock around an insert to form a green body; then removing said insert from the green body thereby forming, in the green body, a cavity having one closed end and one open end and dimensions less than about 1,000 microns; removing all binding materials from said green body, thereby forming a skeleton within which is a micro-channel capable of functioning as a wick; sintering said skeleton thereby forming a dense body that includes said wick; introducing a working fluid into said wick; then sealing said open end, thereby forming said heat pipe; on a surface of said package, forming an open enclosure that is overlapped by said heat pipe; then mounting said integrated circuit chip inside said enclosure on said surface using a high thermal conductivity medium; and then sealing said enclosure.
 19. The process described in claim 10 wherein said high thermal conductivity medium is helium.
 20. The process described in claim 10 further comprising forming, within said enclosure, contact pads and leads, connected to said contact pads, that terminate outside said enclosure.
 21. The process described in claim 10 wherein a high thermal conductivity material is sealed inside said enclosure together with said chip.
 22. A heat pipe, for use with integrated circuits, comprising: a solid body wherein is located a fully enclosed cavity whose diameter exceeds 1,000 microns, said cavity being lined with a porous material; and a working fluid in said cavity.
 23. The heat pipe described in claim 22 wherein said solid body is selected from the group consisting of copper, aluminum, tungsten, tungsten-copper, kovar, stainless steel, and nickel alloys.
 24. The heat pipe described in claim 22 wherein said porous material is selected from the group consisting of copper, aluminum, tungsten, tungsten-copper, kovar, stainless steel, and nickel alloys.
 25. The heat pipe described in claim 22 wherein said working fluid is selected from the group consisting of water, ammonia, acetone, and alcohol.
 26. A package for an integrated circuit chip, comprising: a fully enclosed cavity within a solid body having an upper surface; said cavity being lined with a porous material and containing a working fluid; on said upper surface, overlapped by said cavity, an enclosure within which said integrated circuit chip has been sealed; and said integrated chip being mounted on said upper surface through a high thermal conductivity medium.
 27. The package described in claim 26 wherein said body is selected from the group consisting of copper, aluminum, tungsten, tungsten-copper, kovar, stainless steel, and nickel alloys.
 28. The package described in claim 26 wherein said porous material is selected from the group consisting of copper, aluminum, tungsten, tungsten-copper, kovar, stainless steel, and nickel alloys.
 29. The package described in claim 26 wherein said cavity is between about 2 and 8 mm wide, between about 2 and 5 mm deep, and between about 12 and 60 mm long.
 30. The package described in claim 26 further comprising contact pads, within said enclosure, and leads, connected to said pads, that terminate outside said enclosure.
 31. The package described in claim 26 wherein a high thermal conductivity material is sealed inside said enclosure together with said chip. 